For greatest efficiency, various radio frequency (RF) systems attempt to run power amplifiers at or near their saturation levels, in which the actual output power of the power amplifier is just below its maximum power output level. This power output level is generally related to the supply voltage to the power amplifier, such that a greater supply voltage will produce a correspondingly greater saturated output power from the amplifier. In various prior art power amplifiers, the supply voltage to the power amplifier is fixed. Given a typical usage situation in which actual power output from the amplifier may vary by a range up to several orders of magnitude, use of a fixed supply voltage is highly inefficient as average output power can be an order of magnitude below its maximum and the power amplifier is not maintained at our near its saturation levels.
Various techniques have evolved to vary the supply voltage to maintain the power amplifier at or near saturation. One such technique is using a power supply modulator (PSM) that varies or modulates the supply voltage to the power amplifier to maintain the amplifier at or near saturation while the input signal varies over time. For PSM, the supply voltage of the amplifier tracks input signal variations by typically using a signal detector in conjunction with tracking power supply. PSM, however, introduces nonlinearities in the output signal of the power amplifier, such as gain and phase distortions. While power amplifiers comprised of laterally diffused MOSFET (LDMOS) circuitry have good linearity with respect to input power, LDMOS power amplifiers have variations in gain and phase as a function of supply voltage. When PSM techniques are utilized for amplification efficiency, these nonlinearities cause spectral regrowth in QAM and OFDM modulation systems.
Without use of PSM, there can be less than 5° negative phase variation over a 0-250 mW input power range. Adding PSM adds 15° positive phase variation, which results in a net 10° phase variation over the power range. Experience has shown that the nonlinearities caused without PSM can be reduced to acceptable levels, but such is not the case if the phase variation is 10° or more. Thus, there is a need to reduce the phase variation in PSM power amplifiers.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.